This application is based on German Application DE 1982404.3, filed May 29, 1998, which disclosure is incorporated herein by reference.
This invention relates to an oxidatively aftertreated carbon black for use as pigment in paints, printing inks and inks, for example, for use in ink-jet printers.
Owing to its outstanding properties, carbon black is the main black pigment used in paints and in printing inks. A large selection of pigment carbon blacks having different properties is available. Various processes are used for the production of pigment carbon black. Production is most frequently by oxidative pyrolysis of the carbon-containing raw materials of carbon black. In such a process, the raw materials of carbon black undergo incomplete combustion at elevated temperatures in the presence of oxygen. Examples of this type of process for producing carbon black include the furnace black process, the channel black process and the lampblack process. The carbon-containing raw materials of carbon black used are mainly polynuclear aromatic carbon black oils.
In the furnace black process, the incomplete combustion takes place in a reactor lined with highly refractory material. To this end, a stream of hot waste gas is produced in a preliminary combustion chamber by combustion of a fuel/air mixture and the carbon black raw material is sprayed or injected into this stream of hot waste gas. The carbon black which forms is quenched by water sprayed into the reactor and separated from the stream of gas. The furnace black process permits the production of carbon blacks possessing a very wide range of properties which are useful in practice.
The lampblack and channel black processes are important alternatives to the furnace black process. They yield carbon blacks having properties that overlap to some extent with the useful properties of carbon black which are obtainable by the furnace black process, but they also render possible the production of carbon blacks which cannot be produced through the furnace black process.
The lampblack apparatus consists of a cast-iron shell, which accommodates the liquid or optionally molten raw material, and a fume hood with a refractory lining. The air gap between the shell and fume hood and the partial vacuum in the system serve to regulate the air supply and consequently to influence the properties of the carbon black. The raw material vaporizes as a result of the heat radiation from the fume hood and undergoes partial combustion, but is mainly converted into carbon black. After having been cooled, the process gases containing the carbon black are passed to a filter to separate off the carbon black.
In the channel black process, the carbon black raw material is first of all vaporized into a stream of carrier gas containing steam and then undergoes combustion in a multiplicity of small flames under a cooled cylinder. A portion of the carbon black formed is deposited on the cylinder and another portion is discharged together with the process gases and deposited in a filter.
The important properties for assessing pigment carbon blacks are the blackness value MY (in accordance with DIN 55979), relative tinting strength (preparation of a carbon black paste in accordance with DIN EN ISO 787/16 and evaluation in accordance with DIN EN ISO 787/24), the oil absorption (in accordance with DIN EN ISO 787/5), the volatile constituents (in accordance with DIN 53552), the structure, measured as DBP adsorption (in accordance with DIN 53601 or ASTM D2414), the average primary particle size (by assessment of electron micrographs) and the pH value (in accordance with DIN EN ISO 787/9 or ASTM D1512).
Table 1 shows the ranges of properties of pigment carbon blacks obtainable by the above-mentioned production processes. The data in Table 1 were gathered from technical publications by various manufacturers of carbon black regarding the characteristic carbon black values found for carbon blacks which had not been oxidatively aftertreated.
For a paint or a printing ink, important properties in use are the stability of the carbon black dispersion in the binder system (stability in storage) and the rheological behavior of the paint or the printing ink (viscosity and thixotropy). They are influenced crucially by the chemical structure of the surface of the carbon blacks.
The surface chemistry of the carbon blacks depends greatly on the chosen production process. In the furnace black process, the formation of the carbon black takes place in a highly reducing atmosphere, whereas in the channel black process, the atmospheric oxygen has free access to the zone where carbon black formation occurs. Accordingly, even directly after the production, the content of surface oxides in the channel blacks is considerably greater than in the case of the furnace blacks.
The surface oxides are in the main carboxyl groups, lactols, phenols and quinones, which give rise to an acidic reaction in aqueous dispersions of carbon black. To a lesser extent, the carbon blacks also have basic oxides at the surface. The surface oxides form the so-called volatile constituents of the carbon black, as they can be desorbed from the carbon black surface by calcining the carbon blacks at 950xc2x0 C. (DIN 53552).
The content of volatile constituents has a crucial influence on the dispersibility of the carbon blacks, particularly in aqueous systems. The greater the content of volatile constituents in the carbon blacks, the lesser is the hydro-phobic character of the carbon blacks and the more readily are they dispersed in water-based binder systems.
For the reasons given above, pigment carbon blacks are generally aftertreated oxidatively in order to increase their content of volatile constituents. Nitric acid, nitrogen dioxide and, to a lesser extent, ozone as well, are used as oxidizing agents. The contents of volatile constituents and the pH values given in Table 1 can be increased by oxidative aftertreatment. In this connection, the oxidation behavior depends crucially on the carbon black production process. In the case of furnace blacks, the content of volatile constituents can be increased to only about 6 wt. %. This is as reported in U.S. Pat. No. 3,565,657 regarding the oxidation of furnace blacks by nitric acid. The highest content of volatile constituents given in the said patent is 7.6 wt. %.
In several patents it has been attempted, by ozone treatment of furnace blacks, to reproduce the advantageous properties possessed by channel blacks due to the high content of volatiles in the latter. Among these are the U.S. Pat. Nos. 3,245,820, 3,364,048 and 3,495,999. According to U.S. Pat. No. 3,245,820, the content of volatiles in furnace blacks could be increased to 4.5 wt. % by the ozone treatment.
Another important property of the carbon blacks is their specific surface, which is determined by various adsorption methods. In the determination of the nitrogen surface area (BET surface area in accordance with DIN 66132), one assumes that the surface of the carbon black is covered with nitrogen molecules, the known spatial requirement of the nitrogen molecule rendering possible a conversion into m2/g. As the small nitrogen molecule can also penetrate into pores and crevices in the carbon black, this method also includes the internal surface area of the carbon black. Cetyltrimethyl-ammonium bromide (CTAB) has a spatial requirement larger than that of nitrogen. The CTAB surface area (measured in accordance with ASTM D-3765) therefore comes closest to the determination of the geometrical surface without pores. Hence the CTAB surface area correlates very well with the particle size and thus allows conclusions as to the behavior of the carbon blacks in use.
The iodine adsorption number is a third method of characterizing the surface area of carbon blacks. The iodine adsorption number is measured in accordance with ASTM D-1510. It is greatly influenced by surface groups and by adsorbed PAH""s (=polyaromatic heterocyclic compounds). The adsorption measured in mg/g is not therefore converted into m2/g. Generally the iodine adsorption is only given in the case of carbon blacks containing less than 1.5 wt. % of volatile constituents and less than 0.25 wt. % of a toluene extract. Owing to its sensitivity to the volatile surface groups, however, the iodine adsorption can be used especially as a further possible method of characterizing oxidized carbon blacks having a high content of volatile constituents.
The object of the present invention is to specify carbon blacks for paints and printing inks, which carbon blacks are distinguished by having an improved dispersing behavior in water-based binder systems and by the improved stability in long-term storage of the paints and printing inks produced from them.
This object is achieved by an oxidatively aftertreated carbon black, which is characterized in that its content of volatile constituents is more than 10 wt. %, preferably more than 15 wt. %, based on its total weight, and the ratio of its CTAB surface area to the iodine adsorption number is greater than 2 m2/mg. The ratio of the CTAB surface area to the iodine adsorption number is preferably greater than 4 m2/mg. In addition, these carbon blacks have no measurable concentration of basic surface oxides.
In this connection, the CTAB surface area and iodine adsorption number are measured in accordance with the ASTM standards cited above. It is important here that, prior to the measurement, the carbon blacks are not subjected to a heat treatment for the desorption of the volatile constituents.
It has been found that the consequence of the required combination of properties relating to volatile constituents and to a certain minimum ratio of the CTAB surface area to the iodine value in carbon blacks is that the latter can be very easily dispersed in water and that this dispersion remains stable for days, without the necessity of adding a wetting agent or dispersing additive. This high stability in storage of the aqueous dispersion of carbon blacks renders the carbon blacks according to the invention particularly suitable for use in paints, in printing inks and as inks for mechanical and manual recording and drawing instruments, hence, for example, as inks for ink-jet printers, felt-tip pens and ballpoint pens.
The carbon blacks according to the invention can be obtained by ozone oxidation of channel blacks. Furnace blacks are unsuitable as carbon black starting materials, because their content of volatile constituents cannot be increased to more than about 7 to 8 wt. %, even by ozone oxidation. From appropriate measurements of commercial pigment carbon blacks carried out by various manufacturers, it can be easily shown that the combination of properties of the inventive oxidatively aftertreated carbon blacks was hitherto unknown. Such measurements are reproduced in Table 2.
In this table, the commercial channel black FW 200 is noteworthy. This is a channel black which is not oxidized by ozone. Despite its high content of volatile constituents, it does not exhibit the required ratio of CTAB/iodine.